Along with the advancement of mobile communications, recently, the antenna duplexer is used widely in many hand-held telephones and car-mounted telephones. An example of a conventional antenna duplexer is described below with reference to a drawing.
FIG. 46 is a perspective exploded view of a conventional antenna duplexer. In FIG. 46, reference numerals 701 to 706 are dielectric coaxial resonators, 707 is a coupling substrate, 708 is a metallic case, 709 is a metallic cover, 710 to 712 are series capacitors, 713 and 714 are inductors, 715 to 718 are coupling capacitors, 721 to 726 are coupling pins, 731 is a transmission terminal, 732 is an antenna terminal, 733 is a reception terminal, and 741 to 747 are electrode patterns formed on the coupling substrate 707.
The dielectric coaxial resonators 701, 702, 703, series capacitors 710, 711, 712, and inductors 713, 714 are combined to form a transmission band elimination filter. The dielectric coaxial resonators 704, 705, 706, and coupling capacitors 715, 716, 717, 718 compose a reception band pass filter.
One end of the transmission filter is connected to a transmission terminal which is electrically connected with a transmitter, and the other end of the transmission filter is connected to one end of a reception filter, and is also connected to an antenna terminal electrically connected to the antenna. The other end of the reception filter is connected to a reception terminal which is electrically connected to a receiver.
The operation of an antenna duplexeris described below. First of all, the transmission band elimination filter shows a small insertion loss to the transmission signal in the transmission frequency band, and can transmit the transmission signal from the transmission terminal to the antenna terminal while hardly attenuating it. By contrast, it shows a larger insertion loss to the reception signal in the reception frequency band, and reflects almost all input signal in the reception frequency band, and therefore the reception signal entering from the antenna terminal returns to the reception band pass filter.
On the other hand, the reception band filter shows a small insertion loss to the reception signal in the reception frequency band, and transmits the reception signal from the antenna terminal to the reception terminal while hardly attenuating it. The transmission signal in the transmission frequency band shows a large insertion loss, and reflects almost all input signal in the transmission frequency band, so that the transmission signals coming from the transmission filter is sent out to the antenna terminal.
In this design, however, in manufacturing dielectric coaxial resonators, there is a limitation in fine processing of ceramics, and hence it is hard to reduce its size. Downsizing is also difficult because many parts are used such as capacitors and inductors, and another problem is the difficulty in lowering the assembling cost.
The dielectric filter is a constituent element of the antenna duplexer, and is also used widely as an independent filter in mobile telephones and radio devices, and there is a demand that they be smaller in size and higher in performance. Referring now to a different drawing, an example of a conventional block type dielectric filter possessing a different constitution from the above described structure is described below.
FIG. 47 is a perspective oblique view of a block type dielectric filter of the prior art. In FIG. 47, reference numeral 1200 is a dielectric block, 1201 to 1204 are penetration holes, and 1211 to 1214, and 1221, 1222, 1230 are electrodes. The dielectric block 1200 is entirely covered with electrodes, including the surface of the penetration holes 1201 to 1204, except for peripheral parts of the electrodes on the surface of which the electrodes 1221, 1222 and others are formed.
The operation of the thus constituted dielectric filter is described below. The surface electrodes in the penetration holes 1201 to 1204 serve as the resonator, and the electrode 1230 serves as the shield electrode. The electrodes 1211 to 1214 are to lower the resonance frequency of the resonator composed of the electrodes in the penetration holes, and functions as the loading capacity electrode. By nature, a 1/4 wavelength front end short-circuit transmission line is not coupled at the resonance frequency and shows a band stop characteristic, but by thus lowering the resonance frequency, an electromagnetic field coupling between transmission lines occurs in the filter passing band, so that a band pass filter is created. The electrodes 1221, 1222 are input and output coupling capacity electrodes, and input and output coupling is effected by the capacity between these electrodes and the resonator, and the loading capacity electrode.
The operating principle of this filter is a modified version of a comb-line filter disclosed in the literature (for example, G. L. Matthaei, "Comb-Line Band-pass Filters of Narrow or Moderate Bandwidth"; the Microwave Journal, August 1963). The block type filter in this design is a comb-line filter composed of a dielectric ceramic (for example, see U.S. Pat. No. 4,431,977). The comb-line filter always requires a loading capacity for lowering the resonance frequency in order to realize the band pass characteristic.
FIG. 48 shows the transmission characteristic of the comb-line type dielectric filter in the prior art. The transmission characteristic shows the Chebyshev characteristic increasing steadily as the attenuation outside the bandwidth departs from the center frequency.
In this construction, however, it is not possible to realize the elliptical function characteristic possessing the attenuation pole near the bandwidth of the transmission characteristic, and hence the range of selection is not sufficient for filter performance.
Also, in such dielectric filter, for smaller and thinner constitution, the flat type laminate dielectric filter that can be made thinner than the coaxial type is expected henceforth, and several attempts have been made to design such a device. A conventional example of a laminated dielectric filter is described below. The following explanation relates to a laminated "LC filter" (trade mark) that is put into practical use as a laminated dielectric filter by forming lumped element type capacitors and inductors in a laminate structure.
FIG. 49 is a perspective exploded view showing the structure of a conventional laminate "LC filter". In FIG. 49, reference numerals 1 and 2 are thick dielectric layers. On a dielectric sheet 3 are formed inductor electrodes 3a, 3b, and capacitor electrodes 4a, 4b are formed on a dielectric sheet 4, capacitor electrodes 5a, 5b on a dielectric sheet 5, and shield electrodes 7a, 7b on a dielectric sheet 7. By stacking up all these dielectric layers and dielectric sheets together with a dielectric sheet 6 for protecting the electrodes, an entirely laminated structure is formed.
The operation of the thus constituted dielectric filter is described below. First, the confronting capacitor electrodes 4a and 5a, and 4b and 5b respectively compose parallel plate capacitors. Each parallel plate capacitor functions as a resonance circuit as connected in series to the inductor electrodes 3a, 3b through side electrodes 8a, 8b. Two inductors are coupled magnetically. The side electrode 8b is a grounding electrode, and the side electrode 8c is connected to terminals 3c, 3d connected to the inductor electrode to compose a band pass filter as input and output terminals (for example, Japanese Laid-open Patent No. 3-72706(1991)).
In such a constitution, however, when the inductor electrodes are brought closer to each other to narrow the interval in order to reduce in its size, the magnetic field coupling between the resonators becomes too large, and it is hard to realize a favorable band pass characteristic narrow in the bandwidth. It is moreover difficult to heighten the unloaded Q value of the inductor electrodes, and hence the filter insertion loss is large.
Another different conventional example of a laminated dielectric filter is described below with reference to an accompanying drawing. FIGS. 50(a) and (b) shows the structure of a conventional laminated dielectric filter. in FIGS. 50(a) and (b), 1/4 wavelength strip lines 820, 821 are formed on a dielectric substrate 819. Input and output electrodes 823, 824 are formed on the same plane as the strip lines 820, 821. The strip line 820 is composed of a first portion 820a (L.sub.1 indicates the length of 820a) having a first line width W.sub.1 (Z.sub.1 indicates the characteristic impedance of W.sub.1) confronting the input and output electrodes 823, a second portion 820b (L.sub.2 indicates the length of 820b) having a second line width narrower than the first line width W.sub.1, and a third portion 820c having a third line width narrower than the first line width W.sub.1 but broader than the second line width W.sub.2 (Z.sub.2 indicates the characteristic impedance of W.sub.2). Similarly, the strip line 821 is composed of a first portion 821a having a first line width W.sub.1 confronting the input and output electrodes 824, a second portion 821b having a second line width narrower than the first line width W.sub.1, and a third portion 821c having a third line width narrower than the first line width W.sub.1 but broader than the second line width W.sub.2. The strip lines 820, 821 are connected with a short-circuit electrode 822, and the resonator 801b is in a pi-shape. A dielectric substrate 819 is covered by grounding electrodes 825, 826 at both surfaces. At one side 819a, side electrodes 827, 828 are formed, and the grounding electrodes 825, 826, and short-circuit electrodes 822 are connected. On the other side 819b, side electrodes to be connected with the input and output electrodes 823, 824 respectively are formed. The strip lines 820, 821 are capacitively coupled with the input and output electrodes 823, 824, respectively, thereby constituting a filter as described for example, in U.S. Pat. No. 5,248,949.
In such constitution, however, same as the conventional block type dielectric filter, the elliptical function characteristic possessing the attenuation pole near the passing band of the transmission characteristic cannot be realized, and hence the scope of performance of the filter is not wide enough.